During reproduction of radio programs, tape-recorded programs, and the like, by a car radio it frequently happens that the audio signal reproduced by the loud speakers of the car radio is subject to extraneous noise inteference of widely varying loudness or amplitude. Such extraneous noise is superimposed above the audio program which is intended to be heard, the audio program being entirely or partially masked by the extraneous noise.
Extraneous noise depends on various factors, for example speed of the vehicle, its construction, the quality of noise insulation, the inner appointment of the passenger compartment, the number of passengers in the passenger compartment, ventilation of the passenger compartment, the position of windows, whether open or closed, air-conditioning apparatus including its fan or ventilator, and the like. Any change in any one of the above parameters usually leads the listener to readjust the volume control of the car radio, for increased or decreased volume. If the driver is the only operator of the car radio, this is frequently difficult because the driver has to pay attention to road and traffic conditions, and may require both hands for guidance and control of the vehicle; various operating conditions of vehicles do not permit simultaneously driving and operating a radio button. Additionally, the volume of the reproduced signal varies frequently with respect to the volume of the noise signal so rapidly that manual control is practically impossible.
The audio signal is reproduced with the fidelity characteristic of the electronic reproduction apparatus. High-quality car radios provide high fidelity outputs. Noise during passage of a piano concert, easily masks the particular passage. The volume of reproduction should thus be raised upon acceleration of the vehicle in order to permit the piano passage to be readily perceptible. Even if the noise is at a level such that the piano passage still can be heard, the music sounds distorted.
It has previously been proposed to provide automatic volume control for vehicles in which a controller receives an input based on vehicle speed and/or engine speed. It also been proposed to install a microphone or similar apparatus in the engine compartment of the vehicle in order to measure engine noise and to then cotrol the volume of reproduction as a function of engine noise.
A microphone installed in the engine compartment of the vehicle does not pick up all noises which occur, and referred to above. As a first approximation, however, engine noise may be considered the primary disturbance factor having the widest noise dynamics and the widest rate of change of its amplitude during operation of a car radio.